In the last 15 years, the field of molecular electronics has developed rapidly with the discovery of organic conductive and semiconductive compounds. In this time, a multitude of compounds which have semiconductive or electrooptical properties has been found. It is the general understanding that molecular electronics will not displace conventional semiconductor units based on silicon. Instead, it is assumed that molecular electronic components will open up new fields of use in which suitability for coating large areas, structural flexibility, processability at low temperatures and low costs are required. Semiconductive organic compounds are currently being developed for fields of application such as organic field-effect transistors (OFETs), organic luminescent diodes (OLEDs), sensors and photovoltaic elements. Simple structuring and integration of OFETs into integrated organic semiconductor circuits makes possible inexpensive solutions for smart cards or price tags, which have not been realizable to date with the aid of silicon technology owing to the cost and the lack of flexibility of the silicon units. It would likewise be possible to use OFETs as switching elements in large-scale flexible matrix displays.
All compounds have continuous conjugated units and are divided into conjugated polymers and conjugated oligomers according to the molecular weight and structure. Oligomers are generally distinguished from polymers in that oligomers usually have a narrow molecular weight distribution and a molecular weight up to about 10 000 g/mol (Da), whereas polymers generally have a correspondingly higher molecular weight and a broader molecular weight distribution. However, it is more sensible to distinguish by the number of repeat units, since a monomer unit can quite possibly reach a molecular weight of 300 to 500 g/mol, as, for example, in the case of (3,3″″-dihexyl) quarterthiophene. In the case of a distinction by the number of repeat units, reference is still made to oligomers in the range of 2 to about 20. However, there is a fluid transition between oligomers and polymers. Often, the difference in the processing of these compounds is also expressed with the distinction between oligomers and polymers. Oligomers are frequently evaporable and can be applied to substrates by means of vapour deposition processes. Irrespective of their molecular structure, polymers frequently refer to compounds which are no longer evaporable and are therefore generally applied by means of other processes.
An important prerequisite for the production of high-value organic semiconductor circuits is compounds of extremely high purity. In semiconductors, order phenomena play an important role. Hindrance of uniform alignment of the compounds and development of particle boundaries lead to a dramatic decline in the semiconductor properties, such that organic semiconductor circuits which have been constructed using compounds not of extremely high purity are generally unusable. Remaining impurities can, for example, inject charges into the semiconductive compound (“doping”) and hence lower the on/off ratio or serve as charge traps and hence drastically lower the mobility. In addition, impurities can initiate the reaction of the semiconductive compounds with oxygen, and oxidizing impurities can oxidize the semiconductive compounds and hence shorten possible storage, processing and operating times.
The most important semiconductive poly- or oligomers include the poly/oligothiophenes whose monomer unit is, for example, 3-hexylthiophene. In the linkage of individual or plural thiophene units to form a polymer or oligomer, it is necessary in principle to distinguish two processes—the single coupling reaction and the multiple coupling reaction in the sense of a polymerization mechanism.
In the single coupling reaction, generally two thiophene derivatives with identical or different structure are coupled with one another in one step, so as to form a molecule which then consists of in each case one unit of the two starting materials. After a removal, purification and another functionalization, this new molecule may in turn serve as a monomer and thus open up access to longer-chain molecules. This process leads generally to exactly one oligomer, the target molecule, and hence to a product with no molar mass distribution and a low level of by-products. It also offers the possibility of building up very defined block copolymers through the use of different starting materials. A disadvantage here is that molecules which consist of more than 2 monomer units can be prepared only in a very complicated manner merely owing to the purification steps and the economic outlay can be justified only for processes with very high quality demands on the product.
One process for synthesizing oligo/polythiophenes is described in EP 1 026 138. In the actual polymerization, a regioselectively prepared Grignard compound is used as the monomer (X=halogen, R=substituent):

For the polymerization, the polymerization in a catalysis cycle is started by the Kumada method (cross-coupling metathesis reaction) with the aid of a nickel catalyst (preferably Ni(dppp)Cl2).

The polymers are generally obtained in the necessary purity via Soxhlet purifications.
In EP 1 026 138, the reaction is effected in such a way that first (as quantitatively as possible) the Grignard reaction is prepared and then the thiophene is polymerized with C—C bond formation by adding the nickel catalyst. Similar processes can be found, inter alia, in U.S. Pat. No. 4,521,589 and in Loewe et. al., Advanced Materials 1999, 11, No. 3, p. 250-253 and Iraqi et al., Journal of Materials Chemistry, 1998, 8(1), p. 25-29.
However, the processes described in EP 1 026 138 and in the further literature are purely laboratory processes. For example, in the examples from EP 1 028 136, the reaction solution has a concentration of monomers of about 4-6% by weight and hence a maximum product concentration, for example for the poly-3-hexylthiophene, of 2-3% by weight. The amount of production is increased only by the enlargement of the batch, as described, for example, in Example 2 of EP 1 028 136.
However, such a procedure has the disadvantage that it can frequently barely be used, especially in industrial processes, since the large amounts of solvent frequently prevent an economically viable process. The two-stage process likewise has to be viewed very critically from a safety point of view, since the reaction is started or, better described, ignited a) by the direct addition/mixing of the complete amounts in the Grignard synthesis step or b) also in that case when the catalyst is added.
In many applications, the sole means of control here is through the heat control through the reactor jacket. On an industrial scale, there is the risk of uncontrolled runaway of the reaction owing to the significantly poorer heat removal performance with growing reactor size. In particular, an increase in concentration to enhance the economic viability of the process constitutes a problem in this connection.
Proceeding from the prior art mentioned, it was therefore an object of the present invention to provide a process which at least partly overcomes the disadvantages mentioned and enables the industrial scale preparation of polythiophenes or oligothiophenes with defined mean chain lengths and a narrow molecular weight distribution.